This invention generally relates to vacuum vapor deposition coating of substrates and methods and systems involved in vacuum vapor deposition. More particularly, this invention relates to the production of a highly-active and energized plasma-enhanced vapor from a solid source, such as silicon, and to the application of the plasma within a continuously-operating high-speed coating system.
The plasma-enhanced vapor may be used for deposition onto plastic articles, particularly for depositing a glass-like coating onto plastic bottles. The coating provides an enhanced gas-barrier and better adhesion compared with prior art coatings, and is suitable for pressurized containers, whose surface flexes and stretches, and whose internal pressure acts against an external coating. The primary component of the vapor is produced by evaporating, in an evaporative source, one or more solids and the deposition of the coating may be applied in conjunction with a reactive gas, or gases, to provide desired coating clarity or colorization. Further, it may be produced by using more than one evaporation source and solids of different boiling points.
Commercial applications of plastic articles have experienced a growth, because of the properties of these articles such as low-cost, light weight, flexibility, resistance to breakage, and ease of manufacture and shaping. However, plastics also have the disadvantage of relatively low abrasion-resistance and poor barrier properties against the permeation of vapors such as water, oxygen, and carbon dioxide. In food packaging applications, limitations in barrier properties have limited the use of plastics. For example, in the case of beverage bottles, inadequate barrier properties have restricted the use of smaller bottles required in some markets. Solutions to this problem, including the use of high-barrier plastics and coatings of various types, have been either uneconomical or have provided inadequate barrier-improvement or add expense to the known recycling processes.
A number of processes have been developed for the application of coatings on plastic, but these have been mainly for plastic films. Relatively few processes have been developed which allow the economic application of a glass-like coating onto preformed plastic containers such as PET bottles, where the demands on the coating""s barrier performance are increased by the flexing of the walls of the bottle, the stretching of said walls under pressure, and the delaminating force due to the in-bottle pressure. Also, most processes are on the batch-production principle, and very few processes exist which can be applied to a continuously-running process.
U.S. patent application Ser. No. 08/818,342, now U.S. Pat. No. 6,223,683 filed by Plester et al on Mar. 14, 1997, and PCT International Application PCT/US98/05293 filed on Mar. 13, 1998 describe the use of an anodic arc for externally-coating beverage bottles and their disclosures are incorporated by reference herein in their entirety. Anodic arc systems are also described by Ehrich et al in U.S. Pat. Nos. 4,917,786; 5,096,558; and 5,662,741, the disclosures of which are also incorporated herein by reference.
The basic anodic system, as described by the prior art, has the following disadvantages:
a) The crucibles evaporative material content, such as silicon, cannot be replenished continuously when this evaporative material is in powder or pellet/chip form.
b) The quantity of vapor evolved from the crucible depends partly on the degree of filling of the crucible with evaporative material. Since the degree of crucible-filling is a variable which constantly changes, this could present a control problem.
c) The distribution, at various angular displacements, of the quantity of vapor evolved from the crucible, also depends partly on the degree of filling of the crucible with evaporative material. This makes it difficult to use the vapor from the crucible for the purpose of coating several articles simultaneously, without the risk that these will all receive different amounts of coating.
d) The lips of the crucible are eroded by the anodic arc. This not only presents a maintenance problem, but it also means that the material of the crucible may thus be included in the coating composition and thereby reduce the performance of the coating. For example, crucibles for holding silicon are normally constructed of carbon, which is eroded and vaporized by the anodic arc and the carbon vapor is free to form a contaminant in the desired silicon or silicon dioxide coating.
e) The said crucible lip erosion further affects the quantity of vapor evolved and the distribution of this vapor at various angular displacements around the crucible.
f) Even where the crucible is independently heated (rather than intentionally heated by the anodic arc), the anodic arc represents a second and uncontrolled source of heating. This second source of heating partly affects the quantity of vapor evolved, irrespective of any control device for the crucible""s independent heating system. This makes process control of evaporation rate difficult, whilst evaporation rate is an important parameter.
g) The anodic arc energizes the plasma, but since an uncontrolled and unknown portion of this arc""s energy is dissipated by evaporation of the material in the crucible, this makes the process control of the critical parameter of plasma-enhancement difficult.
h) Since part of the energy of the anodic arc inadvertently causes evaporation, even in anodic arc systems with independent crucible heating, this limits the amount of energy available for plasma enhancement.
i) Anodic arc systems employing independent crucible heating have complicated designs around the crucible in view of the conflicting needs, on the one hand to heat the crucible and on the other hand to provide a cooled anodic connection. This can result in additional cost and complication, oversized heating systems, and energy waste, as well as lead to crucible-damage on shut-down due to the cooling-effect of the anodic connection.
j) Many applications, particularly those involving colored coatings, require the simultaneous evaporation of more than one solid substance. For barrier enhancement, it can also be desirable to add other substances to the base coating. Since such substances differ in boiling point, they cannot be combined in a single evaporating crucible, because evaporative fractionation within the crucible would lead to poor coating composition control. Therefore, multi-component coatings using the anodic arc system must be produced by a multi-series of anode-cathode couples, since one separate anodic arc source for each crucible is needed for process-control purposes. This not only makes a multi-component coating systems complicated and expensive, but also risks interference between the closely positioned array of anodic arcs.
k) The cathode""s evaporative material cannot be replenished continuously and it is therefore desirable in practice to use materials which erode slowly. This acts contrary to the desire to use the cathode for optimum plasma enhancement and ionization, since materials which achieve this often have a high erosion rate. The use of Zn, Cu, Al, noble metals, alkaline earths, and particularly Mg, has been found to be highly desirable, and in most cases continuous cathode replenishment is needed for economic operation.
Prior art exists (German Patent DE 4440521C1, Hinz et al) where the crucible is independently heated by electrical resistance or by thermal radiation, and where the anodic arc plasma-enhancement is provided separately by means of a cathode and a separate anode. However, the anode of such systems quickly becomes coated with the evaporated material from the crucible, or with plasma particles, or with the reaction product when a reactive gas is used. Such systems are therefore only usable where the coating is electrically conducting, since the anode would otherwise quickly become inoperative and the system would shut down. Since the barrier coating of plastic articles often requires the use of coatings with materials such as silicon, which are electrically non-conducting, such prior art cannot be used for many barrier coating systems.
It is important to control accurately the coating thickness on a plastic article and therefore highly desirable to be able to measure continuously, and in situ, the rate of deposition from an evaporative source, so that adjustments to the controls of the said evaporator source can be made as needed throughout the coating operation. Prior art provides means for measuring the rate of deposition by measuring the change of the oscillation frequency of a crystalline substance as the evaporated solids deposit on said crystalline substance. However, the crystalline substance quickly becomes coated and can no longer function, so the system is not usable for normal process control in continuous operating coating systems. A self-regenerating system for rate-of-deposition measurement is needed to enhance process control.
The quality of a coating on plastic articles, particularly the quality of the barrier property of coatings on plastic bottles, is dependent on the control of the degree of ionization and thus on the energy-level of the plasma. A suitably high-energy plasma enables the substrate surface to be cleared of dirt and inert molecules, promoting coating adhesion and coating purity, and further enables coating particles to become embedded in the substrate or to react with the substrate, additionally promoting adhesion. High-energy plasma also promotes the chemical reaction of coating particles with each other, thus forming a dense matrix on the substrate surface, which further enhances adhesion and barrier properties. Finally, high-energy plasma induces coating particles to be deposited in a flat, dense physical structure due to the impingement of high-energy collisions, enhancing coating continuity and denseness. On the other hand, over-energized plasma may overheat the substrate, or cause excessive decomposition or degassing from the substrate, or damage the coating. The evolution of gases from the substrate surface during its degassing mixes with the coating particles and reduces coating quality. It is thus important to measure and control plasma energy and degree of ionization. Prior art does not teach how this can be achieved.
An example of the need for controlled use of high-energy plasma is presented by barrier coating of plastic bottles for carbonated beverages. A barrier coating on a plastic bottle for carbonated beverages must desirably be able to flex, stretch, have adhesion capable of withstanding the pressure migration of the carbon dioxide from the inside of the bottle, and be robust and abrasion resistant in use. It is also desirably dense, preferably amorphous and continuous over the bottle surface. These properties rely on applying controlled. high-energy plasma.
All evaporator systems deposit particles within their enclosure, the latter being normally a high vacuum enclosure. Operation under vacuum is necessary so as to avoid heat damage of heat sensitive substrates such as plastic, and also to avoid gas phase reactions, which in turn would reduce the barrier and other qualities of the coating, since many of these desired properties rely on the on-surface interaction of the coating particles. Particles deposited within the vacuum enclosure tend to disturb the mechanical operation of the coating system and in particular tend to absorb volatiles and make vacuum pump-down more difficult. As a result, the walls of such vacuum enclosures must be cleaned regularly, and this involves production loss and shut-down. An in-situ cleaning system which enables regular and rapid cleaning of the enclosure internals without releasing vacuum and opening the enclosure is desirable for continuous operation and would improve economic operation by reducing downtime.
Accordingly, it is an object of this invention to provide a system for plasma-enhanced evaporation of one or more solid materials, normally inorganic solid material(s), and for use of such an evaporation system in vapor deposition coating of a plastic substrate such as a plastic beverage container, with or without reactive gases, in a manner which enables continuous operation and the provision of a well controlled, high energy plasma. The following are further objects of this invention:
a) To enable replenishment, within the evaporator-crucible system, of the solid material to be evaporated and used for coating without interrupting the evaporator operation;
b) To enable the said evaporator crucible to remain at substantially the same degree of filling during its operation;
c) To provide a vapor particle distribution around the said crucible, which continuously remains constant and well directed;
d) To provide an evaporation system where both the evaporator-energy supply to the crucible and the control of this energy are substantially independent of the energy supplied for plasma-enhancement;
e) To provide an evaporation system with electric arc discharge plasma enhancement which has improved control of each system function, substantially avoids erosion or damage of the evaporator crucible, whose crucible can have a simpler design, which can operate with vapors whose deposited solids are non-conducting electrically, which enables several materials to be evaporated separately but enhanced by the same single arc;
f) To enable continuous replenishment of the cathode""s evaporative material;
g) To enable high energy plasma through use of rapidly eroding materials at the cathode, particularly Mg, other alkali metals, and metals of relatively low boiling point;
h) To enable materials produced by the erosion of the cathode (e.g. Mg, alkaline metals, low boiling point metals. etc.) to be incorporated as dopants in the coating;
i) To enable substantially uninterrupted measurement and control in a continuously running coating process, of evaporation rate and degree of ionization; and
j) To enable in-situ cleaning of vacuum enclosures without need to release vacuum, thereby enhancing the operation of continuously running coating processes.
The foregoing and other objects of this invention are fulfilled by providing a system and method for continuously melting and evaporating a solid material for use in a vapor deposition coating system, a vapor deposition coating system including said continuous melting and evaporating system, a vapor deposition coating system including an electric arc discharge system which switches polarity between electrodes during operation, a vapor deposition coating system comprising an electric arc discharge system including an electrode with combined anodic and cathodic parts for ionization, a continuously fed electrode for producing an electric arc discharge and a coating vapor, a system for measuring the rate of evaporation from an evaporator and the degree of ionization in a vapor deposition coating system, and a self-cleaning vapor deposition coating system. Each of these aspects of the present invention are summarized below.
The system of this invention for continuously melting and evaporating a solid material comprises a melting crucible for receiving and melting a solid material to form molten material and an evaporating crucible for evaporating the molten material. The evaporating crucible is connected to the melting crucible in flow communication with the melting crucible for receiving the molten material from the melting crucible and releasing vapor through an opening in the evaporating crucible as the molten material evaporates. This arrangement allows for additional evaporative solid material to be continuously added to the melting crucible without interfering with evaporation of the molten solid in the evaporating crucible. Accordingly, solid evaporative material can be continuously added to the evaporator during operation of the evaporator so that a coating system incorporating this melting and evaporating system can continue uninterrupted for an extended period. Furthermore, because the evaporating crucible is separate from the melting crucible, the melting crucible and the evaporating crucible can be heated separately and maintained at different temperatures and the evaporating crucible can be made much smaller than the melting crucible. In addition, the evaporating crucible and the melting crucible can be arranged so that the level of molten evaporative material in the evaporating crucible remains substantially constant to provide constant and well directed coating vapor.
The corresponding method of this invention for continuously melting and evaporating a solid material therefore comprises the steps of melting the solid material in a melting crucible to form molten material, flowing the molten material from the melting crucible into an evaporating crucible connected to the melting crucible, evaporating the molten material in the evaporating crucible to form a vapor, and releasing the vapor from the evaporating crucible. This system and method of the present invention desirably includes continuously and automatically feeding the solid evaporative material into the melting crucible as the molten material evaporates so as to maintain the molten material in the evaporating crucible at a substantially constant level during evaporation of the molten material. Various embodiments of this continuous melting and evaporating system include an arrangement wherein the melting crucible and evaporating crucible are arranged so that the melting crucible and the evaporating crucible hold molten material at the same hydraulic level, an arrangement wherein the evaporating crucible draws the molten evaporative solid from the melting crucible via capillary action, and an arrangement wherein the evaporating crucible draws the molten evaporative material from the melting crucible via thermal syphonic force. Other embodiments include an arrangement wherein a pivoting melting crucible melts solid evaporative material and periodically pours molten evaporative material into an evaporation chamber and an arrangement wherein an electrically heated element melts and evaporates solid evaporative material in a melting crucible. Such embodiments do not require energy from an electric arc discharge for evaporation of the solid material and are simple, relatively inexpensive, and resistant to heat damage.
The foregoing system for continuously melting and evaporating a solid evaporative material is particularly useful in a vacuum vapor deposition coating system wherein the continuous melting and evaporating system is disposed within a vacuum cell capable of maintaining a vacuum within the vacuum cell.
The vapor deposition coating system and method of the present invention involving switching polarity between electrodes includes forming a vacuum within a vacuum cell, supplying a coating vapor in the vacuum cell, passing the coating vapor through a gap between a first electrode disposed in the vacuum cell and a second electrode disposed in the vacuum cell, supplying electric power to the first and second electrodes so that the first and second electrodes become oppositely charged and create an electric arc discharge between the first and second electrodes, and switching polarity between the first and second electrodes while the electric power is supplied to the first and second electrodes. The switch is desirably operated automatically and repeatedly switches the polarity between the first and second electrodes and the electric power supply is a DC power supply. By switching the polarity between the first and second electrodes, each electrode alternates between anodic and cathodic function, so that coating vapor, which deposits on the first and second electrodes when the electrodes are in the anodic function, is evaporated when the electrodes are in the cathodic function. Eventually, the coating vapor, when non-electrically conductive, can disrupt the operation of an electrode in a vapor deposition coating system. By switching the polarity between first and second electrodes, the first and second electrodes remain substantially free of deposited coating.
According to another vapor deposition coating system and method of the present invention, a vacuum is formed within a vacuum cell, coating vapor is supplied in the vacuum cell, the coating vapor is passed adjacent an electric arc discharge apparatus, and electric power is supplied to the electric arc discharge apparatus so that a cathode portion of the electric arc discharge apparatus becomes negatively charged and an anodic hood, at least partially covering the cathode becomes positively charged, so that an electric arc discharge is created between the cathode and the anodic hood. The electric arc discharge apparatus includes an electrically insulating material connecting the cathode to the anodic hood, and the cathode and the anodic hood are arranged to form an ionization chamber with the anodic hood having a plasma discharge opening. When electric power is supplied to the electric arc discharge apparatus, an electric arc discharge is created between the cathode and the anodic hood in the ionization chamber, the cathode emits electrons and ionizes the coating vapor disposed in the vacuum cell by the source of coating vapor, the cathode vaporizes and forms an ionized cathode vapor within the ionization chamber and the ionized cathode vapor is emitted from the discharge opening of the anodic hood and mixes with the coating vapor from the evaporation source and the reactive gas, if any, in the vacuum cell to form a coating plasma. The foregoing method and system are relatively simple and economical for producing a plasma enhanced coating vapor in a vapor deposition coating system.
The continuously fed electrode of this invention comprises a plurality of electrode members which vaporize when connected electrically to provide an electric arc discharge, a housing defining a loading chamber for receiving the electrode members in series and including an electrically insulating sleeve, and an electrode member feeder for continuously feeding the plurality of electrode members, in series, through the insulating sleeve in the housing to an electric arc discharge position so that one of the plurality of electrode members is being fed to the electric arc discharge position at a time. This system enables continuous replenishment of the electrodes evaporative material and enables the use of rapidly eroding materials at the cathode of a high energy plasma coating system, enables materials produced by the vaporization of the electrode members to be incorporated as dopants in a vapor deposition coating system, and enables substantially uninterrupted production of ionized vapor in an electric arc discharge vapor deposition coating system.
Desirably, the continuously fed electrode functions as a cathode in an electric arc discharge apparatus. The electrode members are desirably elongate rods or cylinders and are automatically fed from a magazine into the loading housing so that the electrode member feeder can continuously feed the plurality of electrode members, in series, to the electric arc discharge position. In addition, the continuously fed electrode includes a cooling system for cooling the one electrode member, which is in the electric arc discharge position.
The present invention also encompasses an electric arc discharge apparatus comprising the continuously fed electrode described above, an anode, and an electric power supply for supplying electric power to the one electrode member and the anode. The electric power is supplied so that the one electrode member and the anode become oppositely charged with the one electrode having a cathodic charge and the anode having an anodic charge. This creates an electric arc discharge between the one electrode member and the anode so that the plurality of electrode members are vaporized, in series, as each of the plurality of electrode members are fed into the electric arc discharge position within the electrode housing.
Alternatively, the present invention encompasses electric arc discharge apparatus comprising the continuously fed electrode described above and an electric power supply. The continuously fed electrode includes a hood for at least partially covering the one electrode member in the electric arc discharge position. An electrically insulating material insulates the one electrode member in the electric arc discharge position from the hood and the hood is arranged to form an ionization chamber into which the electrode members are fed from the housing. When the electric power supply supplies electric power to the electric arc discharge apparatus, the one electrode member being fed to the electric arc discharge position and into the ionization chamber becomes negatively charged and the hood becomes positively charged so that an electric arc discharge is created between the one electrode member and the hood in the ionization chamber, the one electrode member vaporizes and forms an ionized vapor within the plasma chamber, and the ionized vapor is emitted from the discharge opening of the hood to mix with the vapor from the evaporation source and form a plasma.
The present invention also encompasses a vapor deposition coating system comprising the continuously fed electrode described above and a vacuum cell in which the continuously fed electrode is disposed. This vapor deposition coating system also includes a source of coating vapor disposed in the vacuum cell, a second electrode disposed in the vacuum cell, and an electric power supply for supplying electric power to the one electrode member and the second electrode so that the one electrode member and the second electrode become oppositely charged, create an electric arc discharge and ionize the coating vapor. Desirably, this vapor deposition coating system further includes an evacuation cell for feeding electrode members into the vacuum cell while the vacuum cell maintains a vacuum. The evacuation cell is capable of receiving electrode members from outside the vacuum cell, evacuating air from the evacuation cell, and feeding the electrode members into the vacuum cell under vacuum without disrupting the vacuum within the vacuum cell.
The present invention further encompasses an apparatus for measuring the rate of evaporation from evaporator and the degree of ionization in a vapor deposition coating system comprising two electrical circuits connected to a wire. The first electrical circuit includes a wire, an ammeter connected to the wire for measuring electric current through the wire and a variable DC-power source. When the wire is exposed to an ionized gas, a current flows from the said DC-power source, through the ammeter and through the ionized gas to the walls of the ionized gas enclosure or vacuum cell and to ground. The current flow, measured by the ammeter, bears a relationship to the degree of ionization in the ionized gas and increases as the degree of ionization increases.
The second electrical circuit includes the said wire, a DC or AC supply and a switch. The apparatus desirably includes a timer for controlling the opening and closing of the switch. Particles from the ionized gas deposit on the said wire when the wire is cold and the electrical resistance of theses particles reduces the current flow in the first electrical circuit. When the switch is closed, a current flows within the second electrical circuit and heats up the said wire, thus causing the deposited particles to re-evaporate, which prevents these particles from insulating the wire and affecting the electrical current flow. The electrical current flow measured in the first electrical circuit therefore retains a constant relationship to the degree of ionization, so long as the wire is heated. The measurement of degree of ionization which this relationship provides, can be used to control the degree of ionization, by means of adjusting the current flow from the power supply to the electric arc by conventional means.
When the switch is opened, the wire cools and particles from the ionized gas begin to deposit on the wire. The electrical resistance of these particles reduces the current flow in the first electrical circuit and the rate of reduction bears a relationship to the rate of deposition of particles, which in turn bears a relationship to the rate of production of coating particles by the evaporator and electric arc means. The rate of evaporation can thus be controlled by adjusting the current flow from the power supply to the evaporator by conventional means.
The vapor deposition system itself is as described above and includes an enclosure or cell, which must normally be maintained under vacuum, and a source of ionized coating vapor which is disposed within the said cell. The self-cleaning means includes one electrode, or a plurality of electrodes, disposed within the cell. The electrodes are connected to a power supply and are arranged so that the entire gas space within the cell can be subjected to an ionizing discharge. Suitable forms of power supply include HF, RF and DC. As the coating of substrate proceeds within the cell, it is inevitable that the coating particles deposit also on the interior of the cell and on its internal parts. Such deposits include volatile components which can re-evaporate and impair the function of the coating system. The volatile components of the deposits within the interior of the cell and its internal parts are removed by supplying sufficient ionizing power to the electrode or electrodes disposed in the vacuum cell to ionize gas in the vacuum cell so that the ionized gas removes the deposited coating vapor. This could be done during the operation of the coating system or while the coating system is inoperative.
Other objects, features, and advantages of this invention will become apparent from the follow detailed description, drawings, and claims.